This invention relates to a circuit arrangement for operating a discharge lamp, comprising a self-oscillating DC/AC converter provided with:
a series arrangement of a first and a second switching element coupled between a first and a second input terminal for connection to a DC voltage source, PA1 which series arrangement during nominal lamp operation supplies an alternating current Ib to a load branch which comprises at least first capacitive means, inductive means, and output terminals for connection of the discharge lamp, PA1 a first end of the load branch is connected to a junction point situated in the series arrangement between the first and the second switching element and a second end is connected to an input terminal, PA1 switching elements each of which has a control electrode and a main electrode to which a control circuit is connected equipped with means S and means S' for the generation of a control signal from the alternating current Ib for the first and the second switching element respectively, and PA1 a starter circuit ST comprising means for generating a DC voltage component between the control electrode and the main electrode of the second switching element.
Such a circuit arrangement is known from U.S. Pat. No. 4,748,383. The circuit is designed for an electrodeless discharge lamp provided with a coil and a discharge vessel. During nominal operation, the coil generates a high-frequency magnetic field which maintains an electric discharge in the discharge vessel. The coil is connected to the output terminals of the load branch. The output terminals are shunted by a series arrangement of a capacitor and a primary winding of a transformer. The control circuit of each of the switching elements comprises a secondary winding of the transformer. The primary winding and the secondary windings of the transformer form the means for generating a control signal from the alternating current Ib. The means for generating a DC voltage component between the control electrode and the main electrode of the second switching element are formed by resistive means between the second input terminal and the control electrode of the second switching element and second capacitive means which are connected in series with the means for generating a control signal.
After the circuit arrangement has been switched on, a current flows through the resistive means of the starter circuit to the control electrode of the second switching element, so that the latter enters a conducting state. Then a current flows through the second switching element, through the first capacitive means, and through the primary winding of the transformer. The first capacitive means are charged thereby. The average voltage at a junction point P rises as a result of this from zero to the value which it has during nominal operation. When the DC/AC converter starts oscillating during the charging process, the amplitude of these oscillations is initially so low that the resulting current through the load branch is a variable direct current. While the second switching element is in the non-conducting state, the first switching element must then conduct the current in the reverse direction. A subsequent reversal of the current through the load branch then initiates a recovery interval of the first switching element, such that a peak current arises through the series arrangement. An excessive value of the peak current may lead to damage of the switching elements. To avoid this, it is necessary to choose the capacitance value of the first capacitive means to be comparatively low. A low capacitance value, however, has the disadvantage that the time available for reaching a nominal operational condition is short. It may occur in that case that the first capacitive means have already been charged before an oscillation has established itself. This renders the known circuit arrangement less suitable for operation at comparatively low frequencies, for example at frequencies lower than 100 kHz.
The start of nominal operation may also not establish itself in the case of a too slow rise in the voltage between the input terminals of the DC/AC converter. The strength of the current conducted by the second switching element during charging of the first capacitive means is too low then, and the gain of the switching element is thus insufficient for realizing a satisfactory resonant amplification. If a nominal operational condition fails to arise, the DC/AC converter will end up in a state in which the first switching element is non-conducting and the second one is conducting. In that state there will be no more current flowing through the load branch after the first capacitive means have been charged.